1. Field of the Invention
The present invention generally relates to the fabrication of semiconductor devices. More particularly, the present invention relates to a high-energy ion implanter.
2. Description of the Related Art
In the fabrication of semiconductor devices, an ion implanter is used to modify surface properties of, for example, a semiconductor wafer. More specifically, the ion implanter ionizes dopant atoms and accelerates them to a high speed. Then, a beam of the ions is swept across the wafer surface. The ions penetrate portions of the wafer that are left exposed by a mask. The amount of dopant and the depth of penetration are governed by the size of the dopant atoms, the velocity of the ions, and the amount of time the wafer is exposed to the beam.
An ion implanter may be classified as a medium-current ion implanter, a high-current ion implanter, or a high-energy ion implanter. The medium-current ion implanter generates a beam having a maximum current of about 2 mA at a maximum acceleration energy of about 250 keV. A high-current ion implanter generates a beam having a maximum current of about 20–30 mA and can implant ions at a concentration of about 1014 ions/cm2. A high-energy ion implanter may generate a beam having an energy level of up to several MeV. The high-energy ion implanter helps minimize production costs because its use does not require some conventional processes, such as the masking of the wafer, to be performed. Also, semiconductor devices manufactured through the use of a high-energy ion-implanter exhibit relatively low levels of junction leakage and improved latch-up characteristics. Thus, the production yield is high with respect to an ion implantation process carried out by a high-energy ion implanter. Therefore, high-energy ion implanters are gradually becoming more widely used for implanting ions in a semiconductor device manufacturing process.
FIG. 1 schematically shows a conventional high-energy ion implanter 300. The conventional high-energy ion implanter 300 includes an ion source 321, a vaporizer cell 323, a 110° analyzer magnet 325, and a pre-accelerator (not shown). The conventional high-energy ion implanter 300 further includes a low-energy accelerator 331, a stripper 333, a high-energy accelerator 335, a turbo pump (not shown in the figure), a 10° analyzer magnet 341, and an end station 343.
The ion source 321 creates positive ions from gases that comprise atoms of the desired dopant such as boron (B) or phosphorus (P). The vaporizer cell 323 converts the ionic polarities of the positive ions created in the ion source 321 into desired polarities by using magnesium (Mg). The 110° analyzer magnet 325 separates out only negative ions from the polarized ions. The pre-accelerator applies a proper voltage to the negative ions and thereby accelerates the negative ions.
The low-energy accelerator 331 draws the accelerated ions to a high voltage electrode and then accelerates the ions again for smooth polarity conversion. The stripper 333 removes electrons from the negative ions by making the ions collide with a charge exchanging gas in a vacuum, whereby the negative ions are converted into positive ions. The high-energy accelerator 335 further accelerates the positive ions. The turbo pump creates a vacuum in the stripper 333 and circulates the charge exchanging gas. The 10° analyzer magnet 341 filters the ion beam, accelerated by the high-energy accelerator 335, in an electrostatic manner. The end station 343 implants the ion beam passing through the 10° analyzer magnet 341 into a wafer. The low-energy accelerator 331, the stripper 333 and the high-energy accelerator 335 constitute a tandem accelerator 330.
As discussed above, the ions of the ion beam created in the ion source 321 are in a positive state, and then the ionic polarity of the ion beam is converted in the vaporizer cell 323. The negative ions are separated out in the 110° analyzer magnet 325 and then are directed to the pre-accelerator. In the pre-accelerator, the negative ions are accelerated to a desired energy, e.g., 100 keV, and then are accelerated again in the low-energy accelerator 331 of the tandem accelerator 330. While passing through the stripper 333, the negative ions are deprived of electrons by the charge exchanging gas supplied from the turbo pump and are thereby converted into positive ions. The positive ions are accelerated in the high-energy accelerator 335 and then filtered by the 10° analyzer magnet 341. Finally, the beam of positive ions is implanted into the wafer 10 in the end station 343.
FIG. 2 schematically shows the tandem accelerator 330 of the conventional high-energy ion implanter 300. FIG. 3 is an enlarged view of the stripper 333 of the tandem accelerator 330. Referring to FIGS. 2 and 3, the tandem accelerator 330 includes the low-energy accelerator 331, the stripper 333 and the high-energy accelerator 335 arranged in series and provided within an accelerating tank 351. A high voltage driver 352 and a high voltage multiplier 353 are provided in the upper portion of the tandem accelerator 330 in order to impress a high voltage, e.g., a voltage of 650 keV or 750 keV, across the stripper 333.
The turbo pump 337 is provided under the stripper 333 and supplies the charge exchanging gas to the stripper 333. Furthermore, the charge exchanging gas circulates between the turbo pump 337 and the stripper 333. A dynamo 338 supplies power for driving the turbo pump 337. Because a state of high voltage, e.g., 750 keV, prevails in the accelerating tank 351, the dynamo 338 preferably uses the rotational output of a motor to prevent electric sparks from being produced.
As discussed above, the negative ions are converted into positive ions while passing through the stripper 333. The stripper 333 contains the charge exchanging gas, e.g., N2 or Ar, which reacts with the negative ions and then removes electrons from the negative ions. The resultant positive ions are accelerated in the high-energy accelerator 335, obtaining an additional energy of 650 keV or 750 keV, and thereby attain a high energy of about 1.5M eV.
A low-energy box 361, having a low-energy quadrupole 327 and a low-energy faraday cup 326, is provided at a front end of the accelerating tank 351. In addition, a high-energy box 371 having a high-energy quadrupole lens 340 is provided at the rear end of the accelerating tank 351. The low-energy quadrupole 327, the low-energy faraday cup 326, and the high-energy quadrupole lens 340 serve to focus the ion beam. The low-energy box 361 and the high-energy box 371 also each have a vacuum gauge 381 for measuring the pressure in the box. To this end, each vacuum gauge 381 is capable of measuring high negative pressures.
However, in the conventional ion implanter 300, the charge exchanging gas may flow into the low-energy box 361 and/or the high-energy box 371 due to a malfunction of the turbo pump 337. Such charge exchanging gas may cause an unfavorable rise in the pressure in the low-energy box 361 and/or the high-energy box 371, and may affect the focusing of the ion beam. In that case, the ion beam may collide with electrodes 332 and 336 of the tandem accelerator 330 and lateral walls of the stripper 333. This, in turn, may create contamination, cause accidents and may otherwise adversely impact the reliability of the ion implantation process.
In order to prevent such problems, a conventional high-energy ion implanter 300 not only uses the high vacuum gauges 381 to measure the pressure in the tandem accelerator 330, but also uses sensors to monitor the current, frequency and temperature of the dynamo 338 that drives the turbo pump 337. Accordingly, it is possible to ascertain whether the charge exchanging gas is being circulated properly.
However, the vacuum gauges 381 are only capable of revealing remarkable changes in vacuum pressure. Moreover, the current, the frequency and the temperature of the dynamo 338 may vary with ion implantation conditions. Thus, the sensors may not reveal a slight deviation in the operation of the turbo pump 337 from the intended state of operation. Additionally, monitoring only the turbo pump 337 and associated hardware operation makes it nearly impossible to ascertain whether the charge exchanging gas is flowing into the low-energy box 361 and/or the high-energy box 371 and hence, does not allow for a determination to be made as to whether such an inflow of the charge exchanging gas is affecting the focusing of the ion beam.